1. Field of the Invention
The present invention relates to a semiconductor laser device having an internal current confinement structure and an index-guided structure.
2. Description of the Related Art
(1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 xcexcm band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that the semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp. 102 discloses a semiconductor laser device which emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.48Ga0.52As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, an Al0.2Ga0.8As/In0.2Ga0.8As double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al0.67Ga0.33As etching stop layer, a p-type Al0.48Ga0.52As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al0.67Ga0.33As etching stop layer by the conventional photolithography and selective etching, and an n-type Al0.7Ga0.3As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.
However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.
(2) In addition, IEEE Journal of Selected Topics in Quantum Electronics, vol. 29, No. 6, 1993, pp. 1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.4Ga0.6As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type Al0.4Ga0.6As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al0.4Ga0.6As upper cladding layer by the conventional photolithography and selective etching, and an n-type In0.5Ga0.5P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.
However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having different V-group component, on such an upper cladding layer.
(3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp. 189 discloses an all-layer-Al-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by the conventional photolithography and selective etching, and an n-type In0.5Ga0.5P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.
The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.
(4) Furthermore, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp. 1889-1894 discloses an internal striped structure semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.8 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type AlGaAs lower cladding layer, an AlGaAs/GaAs triple quantum well active layer, a p-type AlGaAs first upper cladding layer, an n-type AlGaAs current confinement layer, and an n-type AlGaAs protection layer are formed in this order. Next, a narrow-stripe groove is formed, by the conventional photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Next, over the above structure, a p-type AlGaAs second upper cladding layer and a p-type GaAs contact layer are formed.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in a fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type AlGaAs second upper cladding layer. However, the above semiconductor laser device also has a drawback that it is difficult to form an AlGaAs layer on another AlGaAs layer since the AlGaAs layers are prone to oxidation.
As explained above, the conventional current semiconductor laser devices which contain an internal current confinement structure, and oscillate at a wavelength of 0.9 to 1.1 micrometers are not suitable for manufacturing and difficult to form a stripe structure with high accuracy. In addition, it is difficult to regrow upper layers after a current confinement layer is formed, when aluminum exists at the regrowth interface, since aluminum is prone to oxidation. Further, even when the upper layers are regrown, the regrowth interface is prone to defect formation. Therefore, the above conventional current semiconductor laser devices are not reliable.
An object of the present invention is to provide a reliable semiconductor laser device which can stably oscillate in an oscillation mode even when output power is high.
According to the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3, and formed on the lower optical waveguide layer (where 0 less than x3xe2x89xa60.4, and 0xe2x89xa6y3xe2x89xa60.1); an upper optical waveguide layer formed on the compressive strain quantum well active layer; a first etching stop layer made of Inx6Ga1xe2x88x92x6P of a second conductive type, and formed on the upper optical waveguide layer (where 0xe2x89xa6x6xe2x89xa61); a second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 of the second conductive type, and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window (where 0xe2x89xa6x1xe2x89xa60.4, and 0xe2x89xa6y1xe2x89xa60.5); a first InxGa1xe2x88x92xP layer of the second conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove (where xxe2x88x920.49xc2x10.01); a current confinement layer made of InxGa1xe2x88x92xP of the first conductive type, and formed on the first InxGa1xe2x88x92xP layer so as to form a third portion of the stripe groove; an upper cladding layer made of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 of the second conductive type, and formed over the current confinement layer and the stripe groove (where x4=0.49y4xc2x10.01, and xxe2x88x920.04xe2x89xa6x4xe2x89xa6xxe2x88x920.01); and a contact layer of the second conductive type, formed on the upper cladding layer. In the above semiconductor laser device, the absolute value of the product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm, and each of layers constituting the semiconductor laser device other than the compressive strain quantum well active layer and the first and second etching stop layers has such a composition as to lattice-match with GaAs.
The first conductive type is different in carrier polarity from the second conductive type. That is, when the first conductive type is n type, and the second conductive type is p type.
The strain of a layer grown above the GaAs substrate is defined as (cxe2x88x92cs)/cs, where cs and c are the lattice constants of the GaAs substrate and the layer grown above the GaAs substrate, respectively. When the absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003, the layer grown above the GaAs substrate is lattice-matched with the GaAs substrate.
Preferably, the semiconductor laser device according to the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The semiconductor laser device may further include at least one tensile strain barrier layer made of Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5 and formed adjacent to the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6. In this case, it is further preferable that the absolute value of a sum of the above product of the strain and the thickness of the compressive strain quantum well active layer and another product of the strain of the at least one tensile strain barrier layer and a total thickness of the at least one tensile strain barrier layer is equal to or smaller than 0.25 nm.
(ii) Each of the lower and upper optical waveguide layers may be one of an undoped type and the first and second conductive types. Layers of the undoped type are not doped with an impurity which produces carriers of a conductive type in the layers.
(iii) The semiconductor laser device may further include a second InxGa1xe2x88x92xP layer of the second conductive type having a thickness of 30 nm or smaller and being formed under the Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 upper cladding layer of the second conductive type so that the second InxGa1xe2x88x92xP layer of the second-conductive type covers the first-conductive type InxGa1xe2x88x92xP current confinement layer and the stripe groove.
The semiconductor laser devices according to the present invention have the following advantages.
(a) The semiconductor laser device according to the present invention can stably oscillate in a fundamental transverse mode even when output power is increased to a high level.
(b) In the semiconductor laser device according to the present invention, the current confinement layer is made of InxGa1xe2x88x92xP, and the upper cladding layer is made of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 of the second conductive type (where x4=0.49y4xc2x10.01, and xxe2x88x920.04xe2x89xa6x4xe2x89xa6xxe2x88x920.01). Therefore, the difference in the refractive index between the current confinement layer and the upper cladding layer realizes a difference of about 2xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923 in the equivalent refractive index of the active layer between the portion under the current confinement layer and the portion under the stripe groove, with high accuracy, and it is possible to cut off oscillation in higher modes. Thus, stable oscillation in a fundamental transverse mode can be maintained even when the output power is increased to a high level. That is, the reliability of the semiconductor laser device is increased by the present invention.
(c) Since the semiconductor laser device includes an internal current confinement structure, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.
(d) The first etching stop layer is made of Inx6Ga1xe2x88x92x6P (0xe2x89xa6x6xe2x89xa61), and the second etching stop layer made of Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 (0xe2x89xa6x1xe2x89xa60.4 and 0xe2x89xa6y1xe2x89xa60.5) is formed on the first etching stop layer. Therefore, when a sulfuric acid etchant is used, only the Inx1Ga1xe2x88x92x1As1xe2x88x92y1Py1 second etching stop layer is etched, and the Inx6Ga1xe2x88x92x6P first etching stop layer is not etched. That is, it is possible to accurately stop etching on the upper surface of the first etching stop layer, and thus the stripe width can be accurately controlled by wet etching.
(e) Since aluminum does not exist at the regrowth interface, it is easy to regrow the upper cladding layer. In addition, since no crystal defect occurs due to oxidation of aluminum, the characteristics of the semiconductor laser device do not deteriorate.
(f) When the compressive strain quantum well active layer is sandwiched between the Inx5Ga1xe2x88x92x5As1xe2x88x92y5Py5 tensile strain barrier layers (where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6), the compressive strain in the quantum well active layer can be compensated for by the tensile strain in the tensile strain barrier layers. Therefore, characteristics of the semiconductor laser device are improved (e.g., the threshold current is lowered), and reliability is increased.
(g) Since the second-conductive type first InxGa1xe2x88x92xP layer is formed under the first-conductive type InxGa1xe2x88x92xP current confinement layer, the difference in the band gap between the second-conductive type Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 upper cladding layer and the layers on both sides of the stripe groove can be increased. Therefore, the spread of the current injected into the semiconductor laser device can be effectively restricted.
(h) When the second-conductive type second InxGa1xe2x88x92xP layer is formed under the second-conductive type Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 upper cladding layer (x4=0.49y4xc2x10.01 and xxe2x88x920.04xe2x89xa6x4xe2x89xa6xxe2x88x920.01) so that the second-conductive type second InxGa1xe2x88x92xP layer covers the first-conductive type InxGa1xe2x88x92xP current confinement layer and the stripe groove, the semiconductor laser device has similar advantages to the semiconductor laser devices described above.